1. Field of the Invention
The present invention relates to a phase difference specifying method, a method of manufacturing a phase shift mask, a phase shift mask, and an exposure method and a method of manufacturing a semiconductor device using a phase shift mask.
2. Background Information
Conventionally, a phase shift mask has been used as a typical photo-mask for use in a photolithographic process in manufacturing a semiconductor device. A phase shift mask is a type of mask which is capable of controlling the phase and transmissivity of light. Accordingly, compared with a chrome (Cr) mask, which is another type of conventional photo-mask, the phase shift mask has an improved printing characteristic and is capable of printing a pattern image on a semiconductor wafer in minute detail.
A typical phase shift mask has a structure in which a film (hereinafter to be referred to a phase shift film) is formed on a predetermined substrate (hereinafter to be referred to as a mask substrate). The phase shift film is made of a material having a different refractive index than the material forming the mask substrate. The phase shift film or the mask substrate has an opening. The phase shift film is arranged to have a refractive index and thickness which enable the optical path difference between the first light penetrating through the opening and the second light penetrating through the other portions thereof to become a half wavelength. In other words, the phase shift film is arranged such that the phase difference between these two lights will become 180°.
In order to measure such phase difference in the phase shift film, i.e. the phase difference between the light penetrating through the opening and the light penetrating through the other portions thereof, a phase difference measuring unit having an interference optical system is used. This phase difference measuring unit uses an inspection light having the same wavelength as an exposure light used in semiconductor device manufacturing processes. On the other hand, there is also a type of phase difference measuring unit which corrects the phase difference in the phase shift film based on the refractive index of the phase shift film with respect to the wavelengths of the exposure light and the inspection light, and therefore, uses an inspection light having a different wavelength than that of the exposure light.
With respect to a phase difference measuring method using the phase shift mask, Laid-Open Japanese Patent Application No. 10-78647 (to be referred to as patent reference 1) discloses related technology, for instance.
According to the conventional technology as disclosed in patent reference 1, a phase shift mask, having a structure in which a backing film (i.e., a chrome film) in which a number of rectangular openings are arranged at predetermined intervals, and a phase shift film in which a number of rectangular openings are arranged at predetermined intervals are formed on the same mask substrate, is used to perform a process of printing an optical image on a predetermined substrate multiple times from different positions in the direction of the optical axis. Then, based on upper and lower limits of the image intensity of the obtained multiple optical images, an amount of focus shift is calculated, and based on the calculated focus shift amount, a phase difference is calculated.
However, one problem with the conventional phase difference measuring method that uses the phase shift mask is that the method requires an expensive measuring unit for exclusive use in measuring optical images. Furthermore, according to the phase difference measuring method disclosed in patent reference 1, the mask position at the time of exposure has to be shifted multiple times in the direction of the optical axis to obtain multiple optical images, which makes the whole process complicated. In addition, when measuring the phase difference using an inspection light having a different wavelength from that of the actual exposure light, it is difficult to correct the phase difference with good precision because of distribution differences in refractive index within the shift film, especially under the conventional method, and therefore the correction result might be greatly influenced by the former possible measurement errors.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved phase difference specifying method, an improved method of manufacturing a phase shift mask, an improved phase shift mask, and an improved exposure method and an improved method of manufacturing a semiconductor device using a phase shift mask. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.